1. Field of the Invention
The present invention relates to DC-to-DC voltage converters, and in particular, to buck converters, and more particularly, to non-isolated buck converters.
2. Description of the Related Art
Voltage converters which convert one DC voltage to another are increasingly used due to their greater voltage conversion efficiency as compared to linear converters. Such converters are typically used to convert unregulated or regulated DC voltage to a different, and sometimes variable, regulated DC voltage at the output. Such converters are widely used in switch-mode DC power supplies and in DC motor drive applications. One common DC--DC converter topology is a buck converter which converts an input DC voltage to a lower output DC voltage and is sometimes called a step-down converter.
Referring to FIG. 1, a common buck converter topology includes an input filter capacitor Cin, a metal oxide semiconductor field effect transistor (MOSFET) Msw, a diode Dfw, an output inductor Lo and an output capacitor Co, all interconnected substantially as shown. The input DC voltage Vdc is filtered by the input capacitor Cin and provides an input DC current Iin. This input current Iin is periodically switched by the switching transistor Msw in accordance with a switching control signal Vc. The alternating on and off states of the switching transistor Msw define a duty cycle D and in accordance therewith provide a switched current Isw. When the switch Msw is turned on, the freewheeling diode Dfw is reverse-biased and, therefore, turned off; and the switched current Isw flows through the output filter inductor Lo as an output current Io which charges the output filter capacitor Co and powers the load Rload. As is well known in the art, the ratio of the output voltage Vo to the input voltage Vdc is equal to the duty cycle D (0&lt;D&lt;1).
When the switch Msw is turned off, the switched current Isw is zero. However, the output current Io through the inductor Lo cannot instantaneously decrease to zero. Therefore, the inductor current Io continues to flow through the loop formed with the load Rload and freewheeling diode Dfw as a diode current Id. The output capacitor Co provides additional charge to maintain the output voltage Vo across the load Rload.
Referring to FIG. 2, a significant problem with this type of buck converter is that of a pulsating input current Iin. The input current Iin fIows during the on state of the switch Msw, but is zero during the off state. This pulsating input current Iin requires that the input capacitor Cin be large so as to handle the ripple current. Additionally, a filter is needed for reducing the electromagnetic interference (EMI) generated by the many high-magnitude signal components at the harmonic frequencies ("harmonics") of the pulsed input current Iin.
The output current Io, due to the low-pass nature of the output filter formed by the output inductor Lo and capacitor Co, is not a pulsating current. However, due to the linear ramping nature of the output current Io waveform (substantially triangular in shape), the output current Io still contains a substantial number of harmonics which can cause interference within the system (load Rload) which is being powered with this output voltage Vo.
One technique which has been used in a variety of ways to address the problems of pulsating currents and high harmonic contents is that of ripple steering. The basic principle behind ripple steering is that the input and output ripple currents are steered in such a way as to reduce or substantially eliminate pulsations in the currents, as well as filtering out much of the harmonic contents of such currents. Examples of ripple steering can be found in U.S. Pat. Nos. 5,038,263 and 5,786,990 (the disclosures of which are incorporated herein by reference).
However, whereas the buck converter topology of FIG. 1 is that of a non-isolated circuit where some form of DC connection exists (at least periodically) between the input and output terminals, the applications involving conventional ripple steering techniques have been in isolated circuit topologies where there is permanent DC isolation between the input and output terminals (e.g., via an isolation transformer). Accordingly, it wouId be desirable to have some forms of ripple steering for non-isolated buck converter topologies.